The invention relates to a semiconductor device and, more particularly, to a method for manufacturing a semiconductor device having bulb-type recessed channels.
With a drastic decrease in the design rules for integrated circuit semiconductor devices, there has been difficulty in ensuring stable operation of transistors. Especially, with a decrease in the design rules for semiconductor devices to a level of 70 nm or less, the size of transistors also has decreased so that the margin of the cell threshold voltage and refresh property reached its limitation.
Under such circumstances, a great deal of research has been conducted on methods for ensuring a longer effective channel length without any increase in the design rules. Among the methods for ensuring a longer effective channel length, an attempt to extend the channel length using a FinFET that introduced a transistor having recessed channels and Fin-shape active regions as structures to extend channel length with respect to a limited gate linewidth has been realized.
However, commercialized high speed double data rate (DDR) 2 DRAM products define test conditions at a high temperature such that the data retention time decreases rapidly compared with the conventional DDR DRAM product. Moreover, deterioration in the transistor drive current characteristics due to the reduced size of active regions in the width direction was also induced. Moreover, a FinFET must remove a isolation layer below a recessed gate compared with a conventional recessed gate, thus the volume of a word line increases. When the volume of the word line increases, the coupling capacitance between the word lines increases greatly thereby inducing a signal delay of the word line. Further, when turning on the word line adjacent to the active regions, the electric field in the junction area is concentrated. Thus, the refresh property may deteriorate due to increased leakage current.
Therefore, a method for minimizing the loss in the isolation layer while ensuring the margin of the cell threshold voltage and improving the refresh property and cell current characteristics by realizing the advantages of a FinFET structure and a transistor structure having recessed channels is demanded.